An estuary is a "partly enclosed body of water" in which river water (fresh water) mixes with ocean water (salt water) (Encyclopedia Britannica Online).
An estuary is an ecosystem that occupies the protected space between
solid land and open water; visually, estuaries often represent the shift
from land to ocean with the presence of a peninsula or barrier islands (EPA, 2007),
although sometimes protective borders are less-visible, like reefs.
Estuaries are protected like rivers, but have tides like the open ocean